Nothing stopping IAG operating Belfast to Gatwick, they have thousands of other slots they could use. The only restriction was they had to give up the slot pairs and DUB/Belfast and they have since had Belfast returned and as soon as the 3 years runs out they can use them on anything.
EU demanded a token gesture, they knew full well IAG had no monopoly on Belfast market but rather London airports slot restricted is whats causing the domestic monopoly by Easyjet and BA at LGW and LHR.